Replication of bacteriophages

Bacteriophages (or simply phages) are viruses that infect bacteria. They replicate inside bacterial host cells, using the host’s enzymes, ribosomes, and metabolic machinery to produce new virus particles.

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Phage replication follows two main types of life cycles:

1. Lytic Cycle – Leads to destruction (lysis) of the host cell.
2. Lysogenic Cycle – The viral DNA integrates into the host genome and replicates along with it.

2. Types of Replication Cycles

A. Lytic Cycle (Virulent Cycle): Carried out by virulent phages such as T4 phage of E. coli. Results in immediate multiplication of the virus and lysis of the host cell.

B. Lysogenic Cycle (Temperate Cycle): Carried out by temperate phages such as λ (lambda) phage. Viral DNA becomes part of the host’s DNA (called a prophage) and replicates passively until induced to enter the lytic phase.

3. The Lytic Cycle: The T4 bacteriophage is a classic model for understanding the lytic cycle.

Step 1: Adsorption (Attachment)

• The phage recognizes and attaches to specific receptor sites on the bacterial cell wall (e.g., lipopolysaccharides or proteins).
• Tail fibers of the phage bind tightly to these receptors.
• This specificity determines the host range of the phage.

Step 2: Penetration (Injection of DNA)

• The phage tail sheath contracts, driving the hollow tail core through the bacterial cell wall and membrane.
• Phage DNA is injected into the cytoplasm, while the empty protein capsid remains outside (the “ghost”).
• The bacterial cell is now infected.

Step 3: Eclipse Phase (Synthesis of Viral Components)

• The phage DNA takes control of the host cell machinery.
• Host DNA is degraded by phage-encoded enzymes (endonucleases).
• The host’s RNA polymerase and ribosomes start synthesizing viral RNA and proteins.

Two types of proteins are produced:

1. Early proteins – enzymes needed for DNA replication (e.g., DNA polymerase).
2. Late proteins – structural proteins for phage head, tail, and fibers.

Simultaneously, phage DNA replicates using host nucleotides.

Step 4: Assembly (Maturation)

• Newly synthesized phage DNA molecules are packed into preformed capsid heads.
• Tail, base plate, and tail fibers are assembled and attached sequentially.
• This process forms complete virions inside the host cell.

Step 5: Release (Lysis)

• Phage produces lysozyme or endolysin, which breaks down the bacterial cell wall.
• The cell bursts (lysis), releasing 50–200 new phage particles into the environment.
• Each released phage can infect a new bacterial cell, repeating the cycle.

Diagram Summary (Lytic Cycle)

1. Adsorption →
2. Penetration →
3. Synthesis →
4. Assembly →
5. Lysis and Release

➡️ Outcome: Destruction of host cell and liberation of new phages.

4. The Lysogenic Cycle (Temperate Phage Replication)

Step 1: Attachment and Entry

• Temperate phage (e.g., λ phage) attaches and injects its DNA into the bacterial cell.

Step 2: Integration

• Instead of replicating immediately, viral DNA integrates into the bacterial chromosome.
• The integrated viral DNA is called a prophage.
• The host cell containing prophage DNA is called a lysogen.

Step 3: Replication with Host

• The prophage is replicated along with the bacterial DNA every time the host divides.
• No new phages are produced during this phase; the virus remains dormant.

Step 4: Induction

• When the host experiences stress (e.g., UV light, chemicals, starvation), the prophage is excised from the bacterial chromosome.
• The phage then enters the lytic cycle, leading to replication and lysis of the host.

Differences Between Lytic and Lysogenic Cycles

6. One-Step Growth Curve

In phage research, the one-step growth curve illustrates stages of viral replication:

1. Latent Period: Time between infection and release of new phages (includes eclipse phase).
2. Burst Size: Number of phages released per lysed cell (usually 50–200).
3. Rise Period: Period during which lysis and release occur.

This experiment shows that phage replication is synchronous within a host population.

7. Significance of Phage Replication

1. Genetic Exchange:
   Phages can transfer bacterial genes through transduction (horizontal gene transfer).
2. Phage Therapy:
   Lytic phages are used to destroy pathogenic bacteria resistant to antibiotics.
3. Molecular Biology Research:
   Understanding replication helped scientists discover gene regulation, DNA replication, and mutation mechanisms.
4. Biotechnology:
   Phage-derived enzymes (like lysozyme and polymerases) are widely used in genetic engineering.

8. Table